The field of the invention is fire retardant, fine particulate, expandable styrene polymers for the preparation of molded articles. The present invention is particularly concerned with expandable, particulate molding compositions of styrene polymers containing organic halogen compounds and an epoxidation product.
The state of the art of expandable polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966) under the section entitled "Foamed Plastics," pages 847-884, particularly pages 852, 853 and 855 where polystyrene is disclosed, and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and pages 120, 121 where prior art self-extinguishing polystyrene foams are disclosed and U.S. Pat. Nos. 3,755,209; 4,029,614; 4,192,922; 4,228,244 and 4,281,036 the disclosures of which are incorporated herein.
The preparation of the epoxidation additives useful in the present invention is disclosed in West German Published Application No. 2,436,817; the article by Swern in Organic Peroxides, Vol. II, pp. 355-533 (1971) and the article by Weigert in Chemikerzeitung #99 at page 109 (1975).
Shaped articles of foamed material are produced commercially by expanding fine particulate expandable styrene polymers in molds. In this procedure, the fine-particulate styrene polymers are first heated with steam or hot gases to temperatures above their softening points and as a result thereof foaming into discrete particles takes place. This process step, wherein the particles have room for free expansion, is called pre-foaming as disclosed in U.S. Pat. No. 4,228,244. The pre-foamed styrene polymers first are stored and then are further expanded in a pressure-resistant mold, which however is not gas-tight, by renewed heating with steam. Due to spatial limitations, the particles fuse together into a molded body corresponding to the cavity of the mold used. This second process step is termed final foaming as also disclosed in U.S. Pat. No. 4,228,244. The molded object, after final foaming, is cooled inside the mold until the inside temperature drops below the softening point. Otherwise, deformation is incurred. The time interval allowing the earliest removal of the molded object from the mold without deformation is ordinarily call the "minimum mold dwell time". It is also possible to use as a measurement, the drop in the inside mold pressure to near atmospheric as a criterion for removing the shaped object from the mold. After being removed from the mold, the molded object most often is stored until fully cooled and thereafter it can be cut into foamed sheets or panels for use as thermal insulation.
Especially when flame retardant organic halogen compounds are added, the production of expandable styrene polymers often results in products having irregular cell structures. Such foamed blocks tend to a significant collapse of their sides (block shrinkage) some time after being removed from the mold, and furthermore they are also less fused inside the block. Consequently foamed panels cut from a block are of varying grades. Furthermore the blocks having defective sides must be trimmed, whereby an undesired waste is incurred.
Another limitation exists in relation to pre-foaming. Part of the pre-foaming beads shrink and accordingly the low densities desired cannot be achieved. The shrinkage of the prefoam beads is related to a high loss in expanding agent, whereby the above mentioned uneven fusing, takes place and hence the collapse of the blocks at the sides is enhanced. Furthermore, the surface of the finished objects has an uneven appearance, which is particularly bothersome for the manufacture of panels which are visible to the public.
U.S. Pat. No. 3,755,209 discloses that by adding hydroxylamines to expandable styrene polymers made self-extinguishing by halogen compounds, it is possible to improve the above cited processing problems. U.S. Pat. No. 4,029,614 discloses a similar effect by adding slight amounts of amines free of hydroxyl groups, and U.S. Pat. No. 4,192,922 defines amine-substituted triazine derivatives for remedying these known processing problems.